1. Field of the Invention
This invention relates generally to methods of preparing ZnSe/ZnS laminates or "sandwiches" by chemical vapor deposition techniques and the laminates so made.
2. Description of the Prior Art
Infrared transmissive windows are currently used at the entrance apertures of many airborne reconnaissance or navigation systems. Materials used for such windows have to satisfy several criteria, including (a) the window should not significantly decrease the incoming signal or render the signal noisy, and (b) the window material should be strong enough to withstand dust and rain erosion encountered during high speed flight. Zinc selenide (ZnSe), a highly transmissive material in the infrared region, satisfies criterion (a) very well and is thus considered an ideal candidate for such window applications. However ZnSe is a "soft" material (hardness about 120 knoop, flexural strength about 8000 psi) which does not effectively resist rain and dust erosion during high speed flight. Zinc sulfide (ZnS), also an infrared transmissive material, does not have the superior optical properties of zinc selenide but is much stronger (hardness about 240 knoop, flexural strength about 15,000 psi), and is capable of withstanding the conditions of high speed flight at least up to about Mach No 1
It is known that a layer of ZnS about 1 mm thick deposited on ZnSe is sufficient to produce a window with rain erosion resistance equivalent to that of pure ZnS without a significant loss in the optical properties of the ZnSe. However, there are problems associated in making such a ZnSe/ZnS "sandwich". One of the main problems is the production of a strong bond between the ZnSe and ZnS layers. Previous attempts at making such a sandwich frequently resulted in products in which the ZnS layer peeled off during polishing and fabrication procedures. Thus, there has been a need for a process by which ZnSe/ZnS sandwiches can be made which have a strongly adherent bond between the ZnSe substrate and the ZnS layer.
One attempt at such a process has been to chemically etch the ZnSe substrate prior to applying the ZnS layer. While the transmission properties of the resulting sandwich were not significantly affected, the strength of the bond between the ZnSe and ZnS layers was not improved.
U.S. Pat. No. 4,303,635, issued Dec. 1, 1981 to Aldinger, et al. discloses optical grade zinc sulfide bodies made by the chemical vapor deposition (CVD) technique. They are subjected to an after treatment under high gas pressure and at elevated temperature. While the CVD technique in general is disclosed, the zinc sulfide bodies are not formed on a ZnSe substrate, so the problem of bond strength is not addressed.
U.S Pat. No. 4,447,469, issued May 8, 1984 to Peters, discloses a low temperature process for depositing, e.g., ZnS on a substrate by reacting a vapor phase reactant (e.g. dimethyl zinc) and neutral, charge-fee sulfur atoms.
U.S. Pat. No. 4,770,479, issued Sept. 13, 1988 to Tutison, and U.S. Pat. No. 4,772,080, issued Sept. 20, 1988 to Tutison generally disclose the production of optical windows of ZnSe or ZnS by the CVD method.
Japanese Patent No. J5 5130-804 disclose ZnSe prepared by reacting hydrogen and selenium vapor to produce hydrogen selenide which is in turn reacted with zinc vapor by a CVD method to produce a ZnSe article.
Japanese Patent No. J5 5130-805 discloses the production of ZnSe by mixing zinc and selenium vapors which causes deposition of ZnSe.
Japanese Patent No. J5 9146-915A discloses a process in which metallic zinc is directed toward a substrate while a sulfur and/or selenium compound gas is supplied, so that ZnS and/or ZnSe is adhered to the substrate.
Japanese Patent No. J5 7118-004 discloses a process in which hydrogen selenide is diluted with an inactive gas and is supplied to a reaction tube in a high temperature furnace. An inactive carrier gas is directed onto molten zinc so that zinc vapor is obtained to grow ZnSe on a substrate.
Japanese Patent No. J6 0169-562-A a method for making infrared transmitting materials of ZnS-ZnSe having excellent adherence to a substrate when subjected to heat cycles. A mixed crystal layer is formed at the interface between the ZnS and ZnSe which is effective in improving the adherence of the ZnS to the ZnSe. The process involves placing a ZnSe board into a CVD synthesis furnace and introducing zinc vapor together with H.sub.2 S gas and H.sub.2 Se gas. The flow rate of the H.sub.2 Se is varied from 100% H.sub.2 SE to 0% H.sub.2 Se (100% H.sub.2 S), after which only M.sub.2 S is present.